I. Field of the Invention
The present invention relates to synthetic silica glass and a method for manufacture thereof. More particularly, the invention relates to synthetic silica glass having high purity which exhibits high viscosities at high temperatures. The inventive glass performs well when made into heat-resistant devices and used in a semiconductor single crystal ingot pulling apparatus.
II. Description of the Prior Art
Crucibles such as those used in single crystal pulling apparatuses, and especially those used in silicon single crystal pulling apparatuses, are commonly made from natural quartz glass because of its excellent heat-resistant property. However, natural quartz glass contains a relatively large amount of impurities which results in serious contamination problems with single crystal ingots. The tendency towards higher integration of semiconductor memories has increased the demand for single crystal semiconductors of higher purity. This has resulted in an increased occurrence of out-of-specification wafers. Attempts have been made to develop methods for making high purity synthetic silica glass that can replace natural quartz glass.
An example of such a method involves hydrolyzing a silicon compound such as silicon tetrachloride in an oxyhydrogen flame to obtain silica grains. After melting the silica grains, the resulting quartz glass is cast in the shape of a crucible, etc. However, a problem with this method is that since as much as 1000 ppm of OH groups are retained in the glass, the viscosity becomes excessively low at high temperatures, and, in the vacuum, the glass foams at high temperatures.
Another example of such method employs a plasma jet flame in place of the oxyhydrogen flame. However, the cost of this method is high and it is difficult to carry out on a mass production basis.
Also, the so-called "sol-gel" method is known wherein an alkoxysilane is hydrolyzed in an alcoholic solvent in the presence of an acid catalyst to obtain silica, which is then sintered to form a synthetic silica glass. Although it is possible to obtain high purity silica glass at a relatively low cost, this method is disadvantageous because of an excessive tendency to retain OH groups and long manufacturing times. Also, it is difficult to obtain silica glass which maintains a high viscosity at high temperature.
The present inventors have developed the following methods based on the sol-gel process for manufacturing synthetic silica glass having high viscosities at high temperatures:
1. Methyl silicate is hydrolyzed in methanol in the presence of ammonia to obtain granular silica having a mean particle diameter of 200-3,000 nm, and this silica is sintered, pulverized and then melted at 1,700.degree. C. (ref. U.S. Pat. No. 4,979,973);
2. Methyl silicate is hydrolyzed in the presence of ammonia to obtain primary granular silica having a mean particle diameter of 100 to 500 nm. This silica is conglomerated to form flocks (secondary granular silica) having a mean particle diameter of 10 to 100 .mu.m, which is separated from the solvent, sintered, pulverized, sieved, melted at 1,700.degree. C. and molded (ref. U.S. Pat. No. 4,979,973);
3. Methyl silicate is hydrolyzed in the presence of ammonia to obtain silica, which is sintered under a reduced pressure at a temperature between 1,500.degree. and 1,700.degree. C., and further sintered under atmospheric pressure or an elevated pressure at a temperature between 1,800.degree. and 2,000.degree. C. (ref. Japanese Patent Application No. 1-139619).
While these methods produce synthetic silica glass having high high-temperature viscosity, they require relatively lengthy process time and high energy cost for the high sintering temperatures required, i.e., 1,500.degree. to 1,900.degree. C.